This invention relates to barrier layer capacitors and more particularly to such capacitors based on donor doped strontium titanate with intergranular type dielectric barrier layers.
Barrier layer capacitors are known which have ceramic bodies that generally include one or many dielectric ceramic barrier layers and include semiconducting ceramic material adjacent to the dielectric barrier layer or layers. The ceramic body is usually sandwiched between two metal electrodes. The apparent dielectric constant of the composite-ceramic body is determined in the usual way from a knowledge of (a) the capacitance as measured at the two metal electrodes, (b) the electrode area and (c) the ceramic body thickness separating the electrodes. The electrical performance characteristics of the capacitor are dependent upon the dielectric properties of the barrier layer or layers, the conductivity of the semiconducting ceramic and the particular geometry of the two. Thus, a barrier layer capacitor may be comprised of a back biased Schottky-barrier layer formed at the interface between a metal electrode and a semiconducting ceramic body. Or a so-called surface-barrier capacitor may be made by oxidizing a thin surface layer of a semiconducting ceramic body and applying a metal electrode over the oxidized dielectric layer. Dielectric barrier layers of a third kind are of particular interest here and are formed at the interfaces between the adjacent grains within the ceramic body. Such bodies are sometimes referred to as grain-interface barrier layer or interfacial polarization types. When barrier layers of the first two kinds are included, the apparent dielectric constant Ka, of the ceramic body is a function of the thickness of the body. But, for capacitors having barrier layers of the third kind, assuming a given grain size, the apparent dielectric constant is not a function of the body thickness, and thus by this test they are similar to the more common ceramic capacitor having entirely dielectric bodies.
Capacitors of the Schottky barrier type and of the surface barrier type may preferably employ a fine grained ceramic body. But, for capacitors of the grain-interface barrier type, large grained ceramic bodies are preferred because for a given thickness body, the average number of dielectric barriers existing between the electrodes is minimized, the apparent dielectric constant increases and the capacity is accordingly enhanced. Methods for making such capacitors include the early step of sintering in a reducing atmosphere in such a way as to grow large semiconducting grains. Subsequently the grain surfaces are exposed to an oxidizing agent and heated in an oxidizing atmosphere to form the dielectric grain-interface barrier layer.
In comparison with the most common type of ceramic capacitors, namely those having a wholly dielectric ceramic body, ceramic barrier layer capacitors can provide much higher apparent dielectric constants while at the same time providing a relatively low temperature coefficient of capacitance. On the other hand, the manufacture of barrier layer capacitors has heretofore entailed the costly, difficult and hazardous step of sintering or reheating in an atmosphere containing active reducing agents, e.g. hydrogen or carbon monoxide.
It is an object of this invention to provide low cost air-sintered grain-interface barrier layer capacitors.
It is a further object of the present invention to provide capacitors of the grain-interface barrier layer type wherein the ceramic body is limited to a particular narrow compositional range leading to excellent electrical properties.